This is already fixed upstream:
https://www.redhat.com/archives/libvir-list/2014-December/msg00800.html
That said, pasting Jan Tomko's suggestion from another bug[1] which is a duplicate of this[2]: update the guest XML config manually to omit those features:
$ virsh edit VM
. . .
Disable the two CPU features:
<cpu mode="custom" match="exact">
<model fallback="forbid">Haswell</model>
<feature policy='disable' name='rtm'/>
<feature policy='disable' name='hle'/>
</cpu>
And a comment from: Josh Boyer 2015-02-10 08:51:03 EST
"As a bit more information, Intel pushed a firmware update that
disabled the rtm/hle operations entirely because the TSX support in
Hgaswell is broken in the chip. It works... until it doesn't. Fedora
includes that microcode update and it is applied during early boot
now."
[1] https://bugzilla.redhat.com/show_bug.cgi?id=1190307 --
3.18.4-200.fc21 (and later) cause guest/host CPU compatibility failures
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1182650 --
Can't launch any VM : rtm and hle CPU features flags needed by
libvirt but not in /proc/cpuinfo
*** This bug has been marked as a duplicate of bug 1182650 ***

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